Illuminated and non-illuminated photodiodes for monitoring and controlling AC and DC components of a laser beam

ABSTRACT

Embodiments of the present invention utilize two photodiodes on the same substrate, one illuminated monitor photodiode to monitor an optical beam out of a back facet (or back scattered) of a laser, and one non-illuminated reference photodiode to characterize in real time radio frequency (RF) parameters/performance to control extinction ratio and optical modulation amplitude of the laser beam.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Divisional of U.S. Ser. No. 10/611,701,filed Jun. 30, 2003.

BACKGROUND

1. Field

Embodiments of the present invention relate to laser systems and, inparticular, to temperature compensation in lasers systems and theirassociated control systems.

2. Discussion of Related Art

One link of an optical telecommunication system typically has atransmitter, an optical fiber, and a receiver. The transmitter has alight source, which converts an electrical signal into the optical beamand launches it into the optical fiber. There is information on a datastream in the electrical signal that is also modulated onto the opticalbeam. The optical fiber transports the optical beam to the receiver. Thereceiver converts the optical beam back into an electrical signal andrecovers the information from the data stream. Laser systems, such asthose that use distributed feedback (DFB) lasers, external cavity lasers(ECL), and vertical cavity surface emitting lasers (VCSELs), are commoncoherent light sources.

To ensure proper operation of any laser system, many of the parameters(e.g., power, channel, temperature) are controlled and monitored bycontrol loops. One such control loop is an automatic power control loop,which is designed to maintain average optical power out of the laserconstant, typically because as lasers age the power output at a givenlaser bias current decreases and as they change temperature, their slopeefficiency changes, resulting in different amounts of light output forthe same bias current.

A typical automatic power control loop includes a monitor photodiodepositioned at the back facet of the laser to monitor the power outputfrom the laser. The light in the optical beam emitted out of the backfacet is either substantially the same as light in the optical beamemitted out of the front facet or has a known proportionality to lightout of the front facet so that the monitor photodiode provides a goodindication of the power in the optical beam light out of the frontfacet. The automatic power control loop adjusts the laser bias currentin response to laser temperature changes and/or aging as sensed by themonitor photodiode.

The light emitted by the laser has what is sometimes called directcurrent (DC) components. The DC component of a high-speed laser isaverage optical power. Average optical power (AOP) is the average levelof power in the optical beam over a time constant much longer than theperiod of one bit of data. However, optical receivers respond to signalswing rather than average optical power, which is not affected in atypical alternating current (AC) coupled laser system by adjusting thelaser bias current in response to laser temperature changes and/oraging.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference numbers generally indicate identical,functionally similar, and/or structurally equivalent elements. Thedrawing in which an element first appears is indicated by the leftmostdigit(s) in the reference number, in which:

FIG. 1 is a high-level block diagram of a laser system according toembodiments of the present invention;

FIG. 2 is graphical representation of an optical beam (or “eye pattern”)according to an embodiment of the present invention;

FIG. 3 is a flowchart showing a method for operating the laser systemdepicted in FIG. 1 according to an embodiment of the present invention;

FIG. 4 is a schematic diagram of a circuit suitable for evaluating thebandwidth of the substrate depicted in FIG. 1 according to an embodimentof the present invention;

FIG. 5 is a schematic diagram of the peak detector depicted in FIG. 1according to an embodiment of the present invention;

FIG. 6 is a schematic diagram of the DC bias extractor depicted in FIG.1 according to an embodiment of the present invention;

FIG. 7 is a high-level block diagram showing operation of the bandwidthprocessor depicted in FIG. 1 according to an embodiment of the presentinvention.

FIG. 8 is a schematic diagram of the temperature processor depicted inFIG. 1 according to an embodiment of the present invention;

FIG. 9 is a graphical representation of the relationship betweentemperature and current for the laser diode given a constant light outdepicted in FIG. 1 according to an embodiment of the present invention;and

FIG. 10 is a high-level block diagram of a communication systemaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1 illustrates a high-level block diagram of a laser system 100according to an embodiment of the present invention. The example lasersystem 100 includes a distributed feedback (DFB) laser 102 that emits anoptical beam 104 from a front facet 106. The laser 102 may be a directlymodulated laser. A lens 108 focuses the optical beam 104 onto an opticalfiber 110. There may be an optical isolator (not shown) positionedbetween the lens 108 and the optical fiber 110. The laser 102 also emitsan optical beam 112 from a back facet 114. In one embodiment of thepresent invention, the optical beam 104 is substantially the same as theoptical beam 112. In an alternative embodiment, the optical beam 104 hasa known proportionality to the optical beam 112.

FIG. 2 is a graphical representation of the example optical beam 112 (or“eye pattern” 200) according to an embodiment of the present invention.An eye pattern is data bits acquired from the data stream in the opticalbeam 112 overlaid on top of each other so that overall quality andstability of a telecommunication system can be observed. Eye patternsallow such parameters as noise, jitter, rise times, fall times, pulsewidths, etc., to be viewed.

In one embodiment of the present invention, the eye pattern 200 isacquired using any suitable known test instrumentation (e.g., anoscilloscope). The particular protocol (e.g., SONET, Ethernet, FibreChannel) used determines how to analyze an eye pattern and associatedtest instrumentation.

The eye pattern 200 includes a direct current (DC) bias level 202, whichis representative of the average optical power in the optical beam 112.The eye pattern 200 includes an amplitude 204, which is representativeof a logic level “1” for the optical beam 112. The eye pattern 200includes an amplitude 206, which is representative of a logic level “0”for the optical beam 112.

Referring back to the embodiment of FIG. 1, the example laser system 100is an un-cooled laser system that includes circuitry to control ACcomponents of an optical beam. One AC component is extinction ratio.Extinction ratio is the ratio of the optical power for the nominal logic“1” and logic “0” levels of an optical beam. Another AC component isoptical modulation amplitude. Optical modulation amplitude (OMA) is thedifference in optical power for the logic level “1” and logic level “0”levels of the optical beam.

The optical beam 112 is incident on a monitor photodiode 116 whoseoutput is coupled to a peak detector 117. The output of the peakdetector 117 is coupled to a bandwidth processor 118. The output of thebandwidth processor 118 is coupled to laser modulation circuitry 122.The output of the laser modulation circuitry 122 is coupled to the laser102.

The monitor photodiode 116 output also is coupled to a DC bias extractor119. The output of the DC bias extractor 119 is coupled to a temperatureprocessor 121. The output of the temperature processor 121 is coupled tolaser bias circuitry 120. The output of the laser bias circuitry 120 iscoupled to the laser 102.

The monitor photodiode 116 along with a reference photodiode 128 arelocated on a substrate 130. The monitor photodiode 116 and the referencephotodiode 128 are physically connected (on the same substrate 130) butare not electrically connected except to similar power supplies.

In embodiments of the present invention, the optical beam 112 does notilluminate the reference photodiode 128. This can be accomplished bypositioning the reference photodiode 128 out of the path of the opticalbeam 112, as illustrated in FIG. 1. Alternatively, the referencephotodiode 128 may have a material on its surface that reflects,absorbs, or otherwise prevents the reference photodiode 128 fromresponding to incident light. In one embodiment, gold or copper aresputtered on the surface of the reference photodiode 128 to reflect theoptical beam 112.

The substrate 130 may be initially tuned for optimal frequency responseat a particular bandwidth. The bandwidth of the substrate 130 changes asa function of temperature, however. As a result, the bandwidth of themonitor photodiode 116 changes as a function of temperature. When thetemperature and as a result bandwidth change, it is difficult toevaluate and control the frequency response of the monitor photodiode116. Embodiments of the present invention adjust the effective frequencyresponse of the monitor photodiode 116 across temperature.

FIG. 3 is a flowchart showing a method 300 for operating the lasersystem 100 according to an embodiment of the present invention. Amachine-accessible medium with machine-accessible instructions thereonmay be used to cause a processor to perform the process 300 or portionsthereof. Of course, the method 300 is only an example process and otherprocesses may be used. The order in which they are described should notbe construed to imply that these operations are necessarilyorder-dependent or that the operations be performed in the order inwhich the operations are presented.

In a block 302, the monitor photodiode 116 converts the optical beam 112to a current proportional to the power in the optical beam 112. Themonitor photodiode current is coupled to the peak detector 117.

In a block 304, the bandwidth processor 118 evaluates the bandwidth ofthe substrate 130, which has a capacitance 403 that is proportional itits (temperature and) bandwidth. Evaluating the bandwidth of thesubstrate 130 also evaluates the bandwidth of the reference photodiode128 and the monitor photodiode 116.

FIG. 4 is a schematic diagram of a circuit 300 suitable for evaluatingthe bandwidth of the substrate 130 according to an embodiment of thepresent invention in which an impulse response measurement (e.g., atime-domain responsivity pulses) is used. The circuit 400 includes an ACtone generator 402 coupled to the substrate 130. The substrate 130 iscoupled to a resistor 404, which is coupled to a choke 406 and ananalog-to-digital converter (ADC) 408. The ADC 408 is coupled to a fastFourier transform tool 410.

In one embodiment, the AC tone generator 402 sends an AC tone throughthe substrate 130. The capacitance 403 shapes the AC tone in accordancewith the bandwidth of the substrate 130. Because both the referencephotodiode 128 and the monitor photodiode 116 are on the same substrate,their capacitances and thus bandwidths are substantially the same asthat of the substrate 130.

The resistor 404 develops the shaped AC tone and the choke 406 shuntsthe DC voltage on the AC tone to ground. The ADC 408 converts the shapedAC tone to a digital value representative of the shaped AC tone. The FFTtool 410 performs a fast Fourier transform on the shaped AC tone so thatthe bandwidth of the substrate 128 can be evaluated in real time.

Returning back to the flowchart 300, in a block 306, the peak detector140 extracts the amplitude for the logic level “1” from the monitorphotodiode 116 current. FIG. 5 is a schematic diagram of the peakdetector 140 according to an embodiment of the present invention.Current from the monitor photodiode 116 is coupled to an AC choke 502,which couples DC components (i.e., DC bias voltage) of the current toground and allows AC components to pass to the rest of the peak detector117 circuitry (e.g., modulation amplitude is coupled to a radiofrequency (RF) diode 504, a capacitor 506, and a low pass filter 508.

The AC choke 502 may be create a low impedance DC path to ground but ahigh frequency open circuit to ground.

The RF diode 504 may be a high-speed Shottky diode that functions as anenvelope or peak detector by essentially clipping negative voltageswings in the AC components.

The capacitor 506 captures the peaks of the negative voltage swings. Thecapacitor 506 value is selected for based on the desired peak detector117 response, i.e., the low-frequency cutoff is selected based on theparticular protocol (e.g., SONET, Ethernet, Fibre Channel) used tominimize pattern dependence.

The filter 508 filters mid to low frequency pattern induced ripples inthe AC portion of the monitor photodiode 116 current. The filter 508 maybe implemented in software, firmware, analog, microcontroller, etc., andafter reading the description herein a person of ordinary skill in therelevant art will readily recognize how to implement the filter 508 in avariety of ways.

The peak detector 117 outputs a voltage proportional to the amplitudefor the logic level “1” of the optical beam 112.

Returning back to the flowchart 300, in a block 308, the DC biasextractor 119 extracts the DC (average) level of the optical signal 112from the monitor photodiode 116 current. FIG. 6 is a schematic diagramof the DC bias extractor 119 according to an embodiment of the presentinvention. Current from the monitor photodiode 116 is coupled to aresistor 602 and a filter comprised of a capacitor 604, an inductor 606,and a capacitor 608. The resistor 602 develops a voltage from thecurrent from the monitor photodiode 116. The filter filters out ACripple in the voltage, leaving a DC voltage level. The DC voltage levelis coupled to an analog-to-digital converter (ADC) 610, which convertsthe analog DC voltage to a digital value.

The output of the DC bias extractor 119 is the DC bias voltage in theoptical beam 112. The DC bias voltage in the optical beam 112 isrepresentative of the average optical power in the optical beam 112.

Returning back to the flowchart 300, in a block 310, measurements aremade from the illuminated monitor photodiode 116 by the peak detector117 and coupled to the bandwidth processor 118. The bandwidth processor118 compensates the output from the peak detector 117 (e.g., amplitudefor logic level “1”) in response to bandwidth variations. FIG. 7 is ahigh-level block diagram showing operation of the bandwidth detector 118according to am embodiment of the present invention. The examplebandwidth processor 118 includes an analog-to-digital converter 702coupled to a microcontroller 704. The microcontroller 704 runs software706, which has a lookup table 708. The lookup table 708 includescalibrated peak detector bandwidth compensations 710 and is indexed fromthe reference photodiode 128's bandwidth measurement.

In a block 312, the adjusted logic level “1” amplitude is coupled to thelaser modulation circuitry 122, which generates laser modulation voltageadjusted in response to the bandwidth variations of the substrate 130.The adjusted laser modulation voltage is applied to the laser 102 toadjust the extinction ratio and/or the optical modulation amplitude ofthe optical beam 104.

In a block 314, the temperature processor 121 derives the temperature ofthe laser system 100 using the reference photodiode 128. FIG. 8 is aschematic diagram of the temperature processor 121 according to anembodiment of the present invention. The temperature processor 121includes a buffer 802 having a resistor 804 coupled across it positiveand negative inputs. A processor 806 is coupled to the output of thebuffer 802. In one embodiment, the processor 806 may include of ananalog-to-digital converter (not shown), and a microprocessor ormicrocontroller (not shown). After reading the description herein, aperson of ordinary skill in the relevant art will readily recognize howto implement embodiments of the present invention using ananalog-to-digital converter and a microprocessor or microcontroller.

The buffer 802 may be a single-ended operational amplifier.Alternatively, the buffer 802 may be a log amplifier. The resistor 804is a sense resistor.

The threshold voltage of the reference photodiode 128 is directlyproportional to the current through the reference photodiode 128 andcurrent changes as a function of temperature. FIG. 9 if a graphicalrepresentation of the relationship between temperature and current forthe reference photodiode 128 according to an embodiment of the presentinvention. The threshold voltage of the reference photodiode 128 thuschanges as a function of temperature.

The reference photodiode 128 has a characteristic curve that illustratesforward voltage versus temperature for the reference photodiode 128. Theconcept of characteristic curves is well known. The temperatureprocessor 121 measures the forward voltage of the reference photodiode128. The processor 806 correlates the forward voltage with the currentusing the characteristic curve. The processor 806 then correlates thevoltage with temperature using the graphical representation in FIG. 9.

Returning back to the flowchart 300, in a block 316, the processor 806adjusts the output from the DC bias extractor 119 (e.g., the DC biaslevel) in response to temperature variations. The processor 806 adjuststhe laser bias level to maintain a constant DC monitor photodiode 116current for a given laser 102 light output over temperature and age. Theprocessor 806 may include a microprocessor (not shown) implementing anysuitable known Proportional-Integral-Differential (PID) loop controlalgorithm to maintain constant monitor photodiode 116 current.

In a block 318, the adjusted peak detector 117 output level (e.g.,adjusted voltage proportional to the amplitude for the logic level “1”of the optical beam 112) is coupled to the laser modulation circuitry122. The laser modulation circuitry 122 generates laser modulationcurrent swing. The laser modulation current swing determines, inconjunction with DC bias current, the amplitude of the logic level “1”in the eye diagram 200. The control voltage that determines themodulation current swing is generated from the bandwidth processor 118and the peak detector 117. The bandwidth processor 118 basicallymaintains (via software control loops) constant optical modulationamplitude (OMA) at the monitor photodiode 116 through this feedbacksystem.

FIG. 10 is a high-level block diagram of a communication system 1000according to an embodiment of the present invention. The system 1000includes a transponder 1002 coupled to an optical amplifier 1004 via anoptical fiber 1006. The optical amplifier 1004 is coupled to amultiplexer 1008 via an optical fiber 1010. The multiplexer 908 iscoupled to a transponder 1012 via an optical fiber 1014.

The transponder 1002 includes the laser system 100. Although only onetransponder 1002, optical amplifier 1004, optical fibers 1006, 1010, and1014, multiplexer 1008, and transponder 1012 are shown, it is common tohave numerous transponders, optical amplifiers, optical fibers, andmultiplexers in optical communication systems. Single units are shownfor simplicity.

The transponder 1002 may transmit optical beams generated by the laser100. Although not shown for purposes of simplicity, the transponder 1002also may receive optical beams from the transponder 1010.

The optical amplifier 1006 may be an erbium (Er) doped fiber amplifier(EDFA). Alternatively, the optical amplifier 1006 may be an ytterbium(Yb) doped fiber amplifier, a praseodymium (Pr) doped fiber amplifier, aneodymium (Nd) doped fiber amplifier, or other suitable opticalamplifier.

The multiplexer 1008 may be a DWDM multiplexer. Alternatively, themultiplexer 908 may be an add-drop multiplexer.

Although embodiments of the present invention are described with respectto a distributed feedback (DFB) laser, it is to be understood thatvarious embodiments may be implemented using vertical cavity surfaceemitting lasers (VCSEL) or other suitable lasers. After reading thedescription herein, persons of ordinary skill in the relevant art willreadily recognize how to implement embodiments of the present inventionusing VCSELs or other suitable (e.g., edge emitting) lasers. Forexample, an optical beam is emitted from a top facet of a VCSEL and isbackscattered from the lens onto the monitor photodiode.

The above description of illustrated embodiments of the invention is notintended to be exhaustive or to limit embodiments of the invention tothe precise forms disclosed. While specific embodiments of, and examplesfor, the invention are described herein for illustrative purposes,various equivalent modifications are possible, as those skilled in therelevant art will recognize. These modifications can be made in light ofthe above detailed description.

In the above description, numerous specific details, such as particularprocesses, materials, devices, and so forth, are presented to provide athorough understanding of embodiments of the invention. One skilled inthe relevant art will recognize, however, that the embodiments of thepresent invention can be practiced without one or more of the specificdetails, or with other methods, components, etc. In other instances,well-known structures or operations are not shown or described in detailto avoid obscuring the understanding of this description.

Various operations have been described as multiple discrete operationsperformed in turn in a manner that is most helpful in understandingembodiments of the invention. However, the order in which they aredescribed should not be construed to imply that these operations arenecessarily order dependent or that the operations be performed in theorder in which the operations are presented.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, process, block,or characteristic described in connection with the embodiment isincluded in at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

1. A method, comprising: converting an optical beam emitted by a laserinto current proportional to a power in optical beam using a firstphotodiode on a substrate, the first photodiode being illuminated by theoptical beam; extracting a direct current (DC) bias voltage level forthe optical beam using the first photodiode; deriving a temperature ofthe laser using a second photodiode on the substrate; preventing thesecond photodiode from being illuminated by the optical beam; adjustingthe DC bias voltage level in response to the derived temperature; andadjusting laser bias voltage in response to the adjusted DC bias voltagelevel.
 2. The method of claim 1, wherein deriving the temperature of thelaser comprises correlating a threshold voltage for the secondphotodiode with a threshold current for the second photodiode.
 3. Themethod of claim 2, wherein deriving the temperature of the lasercomprises correlating a temperature of the second photodiode with thethreshold current for the second photodiode.
 4. The method of claim 1,further comprising adjusting optical signal average optical power basedon the derived temperature.
 5. An apparatus, comprising: a laser; afirst photodiode on a substrate, the first photodiode being illuminatedby an optical beam emitted by the laser; a second photodiode on thesubstrate, the second photodiode being prevented from illumination bythe optical beam; and first circuitry coupled to the first photodiode toadjust direct circuit (DC) components in the optical beam in response tovariations in temperature of the second photodiode.
 6. The apparatus ofclaim 5, wherein the first circuitry includes laser bias circuitrycoupled to adjust laser bias voltage in response to variations intemperature of the second photodiode.
 7. The apparatus of claim 6,wherein the first circuitry is further to adjust average optical powerof the optical signal in response to variations in temperature of thesecond photodiode.
 8. A system, comprising: a transponder having a laserto emit an optical beam, a substrate having a first photodiode and asecond photodiode, the first photodiode being illuminated by the opticalbeam, the second photodiode being prevented from illumination by theoptical beam, and first circuitry coupled to the first photodiode toadjust direct circuit (DC) components in the optical beam in response tovariations in temperature of the second photodiode; and an erbium-dopedfiber amplifier (EDFA) coupled to the transponder.
 9. The system ofclaim 8, further comprising a multiplexer coupled to the EDFA.
 10. Thesystem of claim 9, further comprising an add-drop multiplexer coupled tothe EDFA.
 11. An article of manufacture article of manufacture,comprising: a machine-accessible medium including data that, whenaccessed by a machine, cause the machine to perform the operationscomprising, converting an optical beam emitted by a laser into currentproportional to a power in optical beam using a first photodiode on asubstrate, the first photodiode being illuminated by the optical beam;extracting a direct current (DC) bias voltage level for the optical beamusing the first photodiode; deriving a temperature of the laser using asecond photodiode on the substrate; preventing the second photodiodefrom being illuminated by the optical beam; adjusting the DC biasvoltage level in response to the derived temperature; and adjustinglaser bias voltage in response to the adjusted DC bias voltage level.12. The article of manufacture of claim 11, wherein themachine-accessible medium further includes data that cause the machineto perform operations comprising adjusting optical signal averageoptical power based on the derived temperature.
 13. The article ofmanufacture of claim 12, wherein the machine-accessible medium furtherincludes data that cause the machine to perform operations comprisingcorrelating a threshold voltage for the second photodiode with athreshold current for the second photodiode.